morphology of novel peas containing two consecutive amide bonds randomly distributed along the...

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Morphology of Novel PEAs Containing TwoConsecutive Amide Bonds RandomlyDistributed Along the Polyester Backbone

Priya Garg, Pratiksha Lohakare, Petra Mela, Martin Moller,Bernhard Blumich, Alina Adams*

The morphology of a series of novel segmented PEAs with varying amide content was studiedby a combination of DSC, SFM and proton solid state NMR. Semicrystalline polymers organisedin a lamellar morphology were obtained for all amide contents. The results of different NMRexperiments revealed that a three phase system composed of a rigid phase, an interface and amobile amorphous phase is the most appropriatemodel to describe the morphology of thesematerials. The amount, the chain dynamics andthe domain size of each phasewere estimated andcorrelated with the amount of the amide fraction.Such morphological investigations help to under-stand the macroscopic properties of segmentedPEAs for their applications as biomaterials.

Introduction

Poly(ester amide)s (PEAs) have gained much interest in the

past years as biodegradable polymers for use in biomedi-

cal[1,2] and environmental[3,4] applications. These polymers

combine the favourable properties of polyester (hydrolytic

degradability) and polyamide (high mechanical strength).

Further, by varying the ratio of ester to amide groups, the

physico-chemical properties of PEAs can be tuned in such a

way that they show high degradation rate and/or good

mechanical stability. Depending on the synthetic route and

the monomers used, the microstructure of the PEAs can be

A. Adams, P. Lohakare, B. BlumichInstitute of Technical and Macromolecular Chemistry, RWTHAachen University, Templergraben 55, 52056 Aachen, GermanyFax: þ49 241 802 2185; E-mail: [email protected]. Garg, P. Mela, M. MollerInstitute of Technical and Macromolecular Chemistry, DWI an derRWTH Aachen University e.V., Pauwelsstr. 8, 52056 Aachen, Germany

Macromol. Chem. Phys. 2010, 211, 471–480

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

varied from random,[1,5–9] to alternating[10–14] and to

segmented[15–19] copolymers. Self organisation of the

polymer chain segments in segmented PEAs leads to

the formation of hard and soft domains. Above a certain

amide concentration, the hard domains contain amide

segments which have high melting point and have the

ability to form thermo-reversible physical crosslinks which

contribute to the mechanical stability of the polymer. The

soft domains are built up from ester groups that have a low

glass transition temperature (Tg) and in case of semi-

crystalline ester segments, low melting points.

In spite of the numerous synthetic pathways described in

the literature for obtaining semi-crystalline aliphatic

segmented PEAs, the properties and morphology of

segmented PEAs, where there are always two adjacent

amide groups placed randomly in the polyester backbone,

have received limited attention.[15,20,21] These PEAs have

great potential as biomaterials since the degradation

products are water soluble and hence, no crystalline

remnants are left behind after degradation.[22]

DOI: 10.1002/macp.200900464 471

P. Garg, P. Lohakare, P. Mela, M. Moller, B. Blumich, A. Adams

472

The degradation behaviour as well as the mechanical and

thermal properties of such multiphase polymeric materials

are determined by their microscopic structure not only in

terms of chemical structure and heterogeneity of the chain

dynamics, but also by the spatial arrangement of the

polymer chains in a given morphology. Traditionally, a two-

phase model composed of a rigid and a mobile amorphous

phase is used to describe the morphology of semi-crystal-

line polymers. However, very often, the morphology of such

materials is much more complex and the existence of an

interface between the two phases was reported.[23,24] In the

last decades, the importance of such interfacial region

received particular attention as it affects not only the

thermal but also the mechanical properties of the

material.[25,26] Therefore, detailed knowledge of the micro-

scopic properties in terms of the degree of crystallinity,

phase composition, chain dynamics and domain sizes is

required in order to customise the macroscopic properties of

the material.

Different experimental techniques can be used to

characterise the microdomain structure of multiphase

polymers at the molecular level. They include transmission

electron microscopy (TEM), small-angle X-ray scattering

(SAXS), fluorescence spectroscopy and solid-state NMR

spectroscopy. Among the available techniques, solid state

NMR has emerged as an extremely useful analytical tool for

such investigations. The power of this technique lies in its

ability to obtain various kinds of information by simply

combining different NMR techniques. In particular, it can

probe the microphase separation and extract domain

thickness with the help of spin diffusion experiments. This

type of experiment, in which the proton magnetisation is

selectively suppressed in one of the phases with the help of

dipolar filters[27–30] and then allowed to equilibrate by

means of dipolar-mediated flip-flop process was success-

fully applied to various types of solid polymers including

polyolefins,[24] nylon fibres[29,30] and block copolymers,[29–32]

but also to much more complex systems such as confined

biomolecules.[33] Generally, the domain sizes estimated by

the proton spin diffusion agree very well with those

estimated by X-ray diffraction.[24]

In the case of PEAs, Serrano et al. applied various 13C solid-

state NMR techniques to study the morphology of

alternating PEAs.[34] They showed that the length of the

spacer has significant impact on the molecular dynamics.

Moreover, a two-domain structure composed of amorphous

and crystalline regions has been identified for such systems.

Furthermore, Aharoni and coworkers used solid state

NMR under magic angle spinning (MAS) conditions for the

structural characterisation of liquid-crystalline PEAs.[35,36]

They showed that the methylene groups between the

amide linkage are in trans conformation while the ester

methylene groups exhibit both gauche and trans con-

formations depending on the length of the spacer group.



Generally, 13C solid-state NMR represents a powerful

technique for the characterisation of the multiphase

polymer systems but the main disadvantages are related

to the long experimental time due to the low 13C sensitivity

and to the non-quantitative cross-polarisation transfer. On

the other hand, proton NMR gained significance as a reliable

tool for the characterisation of multiphase polymer

materials, even for those with complex morphologies as

long as sufficient molecular mobility exists between the

different phases in order to discriminate them.[24,29,30]

Proton NMR benefits from high sensitivity which translates

into short measuring time and from simplicity in perform-

ing the experiments without the need of expensive and

complicated NMR probes.

Here, a combination of different techniques such as

proton solid state NMR, differential scanning calorimetry

(DSC) and tapping-mode scanning force microscopy (SFM)

was applied to investigate the morphology of aliphatic

segmented PEAs containing two adjacent amide groups

with varying amide to ester ratio. Special attention was

paid to the estimation of the domain sizes with the aim of

establishing a correlation between the microscopic and

macroscopic properties necessary for the understanding

the macroscopic behaviour of these materials. To our

knowledge, this is the first time that such detailed

morphological information is reported for this type of PEAs

samples.

Experimental Part

Materials

A two-adjacent amide series of PEAs with varying ratios of amide to

ester groups was prepared by a 2-step polycondensation reaction as

described elsewhere.[37] The resultant polymers are named as PEA-

x% with PEA for PEA and x% (10–65%) for the theoretically

incorporated amide segment concentration. The composition and

the molecular weights of the synthesised PEAs together with those

of the homopolyester poly(butylene adipate) (PBA) are summarised

in Table 1. All solvents were obtained from Fluka, Germany and

used as received. Silicon wafers were obtained from CrysTec GmbH,

Germany.

Instrumentation

Differential Scanning Calorimetry

DSC was conducted on a Netzsch DSC 204 unit equipped with a

liquid nitrogen cooling accessory unit (Netzsch CC200 supply

system). Indium was used as a calibration standard. The samples

(typical weight, 5–7 mg) were enclosed in standard Netzsch 25mL

aluminium crucibles. Unless otherwise stated, all measurements

were carried out at heating and cooling rates of 10 K �min�1 from

�75 to þ300 8C under a continuous nitrogen flow of 50 mL �min�1.

Three successive runs (heating/cooling/heating) were performed

DOI: 10.1002/macp.200900464

Morphology of Novel PEAs Containing Two Consecutive Amide Bonds . . .

Figure 1. (a) General scheme for a spin diffusion experiment usingan MQ dipolar filter. (b) Pulse sequence used for the spin diffusionexperiments using a DQ dipolar filter.

Table 1. Composition and molecular weight of the synthesisedpolymers.

Polymer m/nth. m/nexp.a)

Mnb) Mw

b) PDI

mol-% mol-%c) Da Da

PBA 0/100 0/100 17 700 34 000 1.93

PEA-10% 10/90 11/89 19 000 38 000 1.99

PEA-25% 25/75 27/73 16 500 33 200 2.00

PEA-40% 40/60 41/59 14 300 30 000 2.10

PEA-50% 50/50 49/51 13 000 26 800 2.05

PEA-65% 65/35 63/37 7 500 14 800 1.98

a)n and m refer to the percentage of amide and ester repeating

units in the PEA, see Scheme 1 below; b)From SEC measured in

dimethylformamide, Mn: number-average molecular weight, Mw:

weight-average molecular weight; c)From 1H NMR measured in

CDCl3 or trifluoroacetic acid-d.[37]

for each sample. The cooling and second heating runs are reported

here.

Scanning Force Microscopy

SFM was performed with a Nanoscope III microscope (Digital

Instruments, Veeco, Santa Barbara, USA). Investigations in the

tapping mode were carried out with silicon cantilevers from

Nanosensors (Wetzlar, Germany) with a spring constant of

�50 N �m�1 and at a tapping frequency of about 350 kHz. Images

were edited with Nanoscope software (v5.12r5 Digital Instruments,

Veeco, Santa Barbara, USA). Height and phase images were

recorded at various magnifications.

A solution of the polymer in HCOOH (5 mg �mL�1) was spin

coated on an ozone activated silicon substrate. The substrate was

then heated to 200 8C on a Mettler Toledo hot stage FP82HT

equipped with a FP90 central processor. The molten film was

annealed for 5 min at 200 8C and cooled to room temperature at

5 8C �min�1. Subsequently, SFM was performed on these films.

Solid-State NMR

Proton NMR measurements have been performed using a Bruker

DSX-200 spectrometer working at a frequency of 200.12 MHz for

protons. A dedicated solid-state probe body without proton NMR

background was used for the wide-line, double-quantum (DQ)

filtering and spin diffusion experiments. The data were collected for

static samples at room temperature. The duration of the 908 pulse

was 1.5ms and a recycle delay of 3 s was used for all experiments.

The experimental wide-line spectra were decomposed in three

components using the DMFIT program.[38] The broad component of

the spectra could be well approximated by a Gaussian function

while the narrow line was fitted with a Lorentzian function. A

combination of the two functions was employed for the

component with intermediate mobility. In order to simplify data

analysis, the number of fit parameters were reduced by keeping the

positions of the signals and the line widths fixed based on the

results from the DQ filtering experiments.



Proton DQ filtering and the spin diffusion experiments were

recorded using the five-pulse sequence shown in Figure 1. In the

case of the filtering experiments the excitation/reconversion times

t were varied in the range of 1–500ms with a fixed spin diffusion

time of 5ms. For the spin diffusion experiments an excitation/

reconversion time t¼7ms was used in order to mainly select the

magnetisation from the rigid phase. The evolution time of the DQ

coherences (tDQ) was 5ms in all experiments. The values of the

estimated longitudinal relaxation times for all investigated

samples are much longer than the time necessary for the

magnetisation to equilibrate within the spin diffusion experiments

and therefore, no correction of the spin diffusion data due to

relaxation effects was needed.

Results and Discussion

Polymer Synthesis

The amide containing a,v-amino alcohol monomer (3) was

synthesised by ring opening of e-caprolactone (2) with 1,4-

diamine (1) as shown in Scheme 1. Melt polycondensation

of this preformed monomer with 1,4-butanediol (5) and

dimethyl adipate (4) resulted in segmented PEAs with two

adjacent amide groups randomly incorporated in the PBA

backbone. The ratio of the preformed a,v-amino alcohol (3)

to 1,4-butanediol (5) could be varied in order to yield PEAs

with varying amide content. Although the a,v-amino

alcohol (3) is directional and two orientations of this unit in

the PEA chain would be feasible, the two adjacent amide

groups are symmetrical. Thus, we believe that the

orientation of the monomer unit in the PEA chain is not

relevant for its properties. The molecular weight distribu-

tion, as determined by means of SEC, shows that most of the

synthesised polymers have a molecular weight of

14 800<Mw <30 000 (Table 1). The molecular weight

decreased as the amide content in the PBA backbone

increases. The polydispersity index (PDI) is close to 2

www.mcp-journal.de 473


(m) H2N-(CH2)4-NH-CO-(CH2)5-OH + (m+n) H3C-O-CO-(CH2)4-CO-O-CH3 + (n) HO-(CH2)4-OH

CO-(CH2)4-CO-NH-(CH2)4-NH-CO-(CH2)5-O* co CO-(CH2)4-CO-O-(CH2)4-O *m nPEAru PEru

(3) (4) (5)

Poly(ester amide)

Ti (IV) isopropoxide170 °C, 5 hTransesterification

Oligomers

180 °C, 4 hMelt polycondensation

H2N-(CH2)4-NH2 +O

O

H2N-(CH2)4-NH-CO-(CH2)5-OH

(1) (2) (3)

Scheme 1. Synthesis of a,v-amino alcohol (3) and subsequent polycondensation with 1,4-butanediol (5) and dimethyl adipate (4). PEAru denotes PEA repeat unit and PErupolyester repeat unit.

474

(Table 1). The microstructure of these polymers could be

evaluated by NMR (see Table 1).[37] The theoretical and

experimental compositions of the synthesised PEAs

(Table 1) are in good agreement, indicating that the

polymerisation reactions were successful. More detailed

information about the synthesis and characterisation of the

obtained products is given elsewhere.[37,39]

Thermal Properties

The thermal properties of the PEAs were determined using

DSC. Figure 2 shows the variations of the transition

temperatures Tg, melting temperatures Tm and the

corresponding enthalpies as a function of the amide

content in the PBA backbone. It can be clearly observed

that all samples are semi-crystalline. Further, with the

addition of the amide groups to the PBA chain, the

enthalpies of melting and crystallisation decrease initially

from PBA to PEA-25% and then increase with decrease in

ester content until PEA-65%. A detailed study of the thermal

properties of these polymers is presented elsewhere.[40]

Visualisation of the Morphology by SFM

SFM represents an alternative to TEM[41,42] for the

visualisation of the microphase structure of segmented

polymers. It has been used successfully to elucidate the

morphology of segmented polyurethanes[41–43] and other

block copolymers.[44–46] In order to investigate the surface

morphology of the two-adjacent amide series of PEAs,

tapping-mode SFM was applied to melt crystallised

samples. For illustration, the SFM images of PEA-10, 25



and 65% are shown in Figure 3. The

images (a), (c) and (e) are the height

images and the images (b), (d) and (f) are

the corresponding phase images.

These samples clearly show a micro-

phase separated morphology. In the

phase image, hard and soft domains

appear as light and dark regions, respec-

tively. Oriented crystallites are observed

which have long ribbon-like structures

with thicknesses of a few nanometres

and lengths of several micrometres. The

crystallites, therefore, display high aspect

ratios. Furthermore, these ribbons are

well dispersed and form interconnected

structures which are embedded in the

amorphous matrix. It is important to note

that the resulting morphology of these

polymers is lamellar. This information

will be used for determining the domain

sizes of the different phases in the

polymers by solid state NMR.

Solid-State NMR

Phase Composition and Chain Dynamics from 1H NMR

It is well known that the line width of the 1H line reflects the

nature of the dipolar interaction between the protons and

can be used to monitor the dynamic behaviour of the

polymeric chains.[27,47] For the chains in the crystalline and

in the glassy state, the mobility is restricted, and the dipolar

couplings are on the order to 30–50 kHz. On the order hand,

the chains in the amorphous phase at a temperature above

Tg show higher mobility, and the dipolar couplings are

partially averaged. Therefore, at temperatures above the Tg

it is possible to discriminate between the various phases by

proton NMR measurements performed without MAS.[24,30]

However, the crystallinity values extracted by NMR

generally differ from those obtained by other methods

for different reasons: (i) the crystalline and amorphous

phases are discriminated on the basis of different proper-

ties, such as the enthalpy of melting in DSC and chain

mobilities in NMR. (ii) The simplified two-phase model

often is not the most appropriate way to describe

the morphology of semi-crystalline polymers due to the

presence of a crystalline-amorphous interface. In the case of

NMR measurements, this interface may be counted either

as crystalline or amorphous fraction depending on the

temperature where the measurement is performed. There-

fore, it is more appropriate, to speak about a rigid phase

than a crystalline phase. Moreover, as the definition of the

three phases is related to the NMR time scale of the chain

DOI: 10.1002/macp.200900464


Figure 2. Variation of (a) the glass transition temperature Tg,(b) the melting temperature Tm and the corresponding enthalpyand (c) the crystallisation temperature Tc and the correspondingenthalpy with the amide segment content.

dynamics different methods may provide different values

not only for the rigid phase but also for the interface and the

amorphous phase.

A typical single-pulse proton spectrum obtained under

static conditions is depicted in Figure 4. The shape of the

spectrum points towards the presence of at least two

different phases within the sample. However, the best fit of

the experimental data was obtained by using a combina-

tion of three components as described in the Experimental

Part. The broad component at the bottom of the spectra

corresponds to the rigid phase while the narrow component

corresponds to the mobile amorphous phase. The third



component has a mobility in between that of the rigid and

mobile phases and therefore, this phase should correspond

to an interface. The existence of the interface is confirmed

(see below) by the filtering and by the spin diffusion

experiments.

The DSC results give evidence that all the samples

investigated in this work are semi-crystalline. In the case of

PBA, the broad component corresponds to the rigid PBA

fraction which includes the crystalline phase as well as a

part of the interface while the narrow component

corresponds to the chains in the mobile amorphous

fraction. By incorporating the amide fraction in the PBA

chain, the morphology of the PEA system becomes more

complex. High-resolution 13C solid-state NMR measure-

ments under MAS revealed that at low amide content (10%),

the crystalline fraction is made from the PBA units, whereas

at higher amide content (�25%), the crystalline phase is

represented by an amide-rich phase with a mobile

amorphous phase represented by the mobile amorphous

PBA fraction.[48]

The variation of the amounts of the three different

phases with the amide segment content is shown in

Figure 5a and the corresponding line widths are depicted in

Figure 5b. The PBA homopolymer has a high amount of the

rigid phase. In PEA-10% the amount of the rigid phase is less

than in the PBA which can be attributed to the disruption of

the crystalline structure of the homopolyester by the

introduction of the amide fraction. For the polymers with

the higher amide content (PEA-25% to PEA-65%) the amount

of the rigid phase is lower than that of PBA and it increases

with increase in amide fraction. On the other side, a

decrease in the amount of the mobile amorphous phase and

an increase in the amount of the interface is observed with

increase in amide content.

The effect of the amide content on the chain mobility of

the three phases is reflected on the linewidths, as shown in

Figure 5b. The mobility of the rigid phase is most restricted

in the homopolyester. With the introduction of the amide

segments, an increase in the chain mobility of the rigid

phase is observed. The most mobile rigid phase corresponds

to the PEA-25% which is due to the disruption of the ordered

structure of PBA with the introduction of the amide groups.

However, a slight increase in the rigidity with increase in

amide content is then observed for PEA-25% to PEA-65%

which may be related to the formation of an increasing

number of H-bonds between the amide segments in the

crystalline regions. Increasing amide content (>25%) has,

on the other hand, only a small effect on the chain dynamics

of the interface when comparing PBA, PEA-10% and PEA-

25%. However, a stronger effect on the chain dynamics of

the mobile amorphous phase was observed due to the

incorporation of the amide groups. The chain mobility

decreased with decreasing ester content and this is in

agreement with the variation of Tg (Figure 2a).



Figure 3. SFM images of melt crystallised samples of (a), (b) PEA-10%, (c), (d) PEA-25% and(e), f) PEA-65% taken at room temperature: (a), (c) and (e) are height images and (b), (d)and (f) are phase images.

Figure 4. Experimental and theoretical 1H NMR spectrum of PEA-65%. The broad and narrow lines correspond to the rigid andmobile components, respectively, and the intermediate line cor-responds to the interface. The measurements were performed atroom temperature.

Figure 5. Variation of (a) the phase composition and (b) thelinewidths from the different phases of PEAs with the amidecontent.

476

Heterogeneous Dynamics from Proton DQ-Filtered NMR

In the last years, the proton dipolar filter based on the

excitation of the DQ coherences was successfully used in

spin diffusion experiments performed under static and

MAS conditions for the selection of the magnetisation from

the rigid phases in various multiphase polymer materi-

als.[24,29,49,50] The most prominent reason for its use is the

fact that in the spin diffusion experiment a narrow signal is

monitored to grow on top of a broad component which can

be quantified with better accuracy.[29] Furthermore, the DQ

filter can provide detailed information about the structure,

heterogeneous dynamics, and miscibility in polymer

blends.[29,50] Moreover, it can be used in combination with



other types of dipolar filters which select

the magnetisation from the mobile phase

for obtaining information about the

dimensionality of the spin diffusion

process.[29]

By appropriate selection of excitation/

reconversion periods of the multiple-

quantum (MQ) coherences, the magneti-

sation of the domains with stronger

dipolar couplings in a heterogeneous

sample will pass through the filter

whereas that of domains with the weaker

dipolar couplings will be filtered out.

Generally, at short excitation/reconver-

sion times, the DQ filter selects predomi-

nantly the magnetisation from the rigid

phase.[29] At longer excitation times, the

magnetisation from the mobile amor-

phous phase can be selected as the DQ

dipolar filter starts to act as a T2 filter

and the magnetisation from the rigid

phase decays to zero.[29] The filter

shows excellent efficiency in selecting

the magnetisation arising from different phases. Therefore,

the peak shape and its line width as a function

of the excitation/reconversion times can provide useful

DOI: 10.1002/macp.200900464


Figure 6. 1H spectrum and the DQ filtered 1H NMR spectra of PEA-65% at varyingexcitation/reconversion times measured using the pulse sequence depicted in Figure 1.

information about the dynamics heterogeneity of the

polymer segments.

Figure 6 shows the effect of increasing the excitation/

reconversion time t on the spectrum of PEA-65%. In

Figure 6a the proton spectrum obtained without the filter

shows broad and narrow components. At short t, i.e., at

values before the maximum of the DQ build-up curves

(10ms here) the narrow components are suppressed and

only the signal from the broad component is observed

(Figure 6b). The doublet lineshape is the signature for the

presence of strong dipolar interactions in this phase. At

longer t (200ms, Figure 6d) only a narrow signal of about

3 kHz width can be seen which belongs to the mobile

amorphous phase. At these long values of t, the single

quantum coherences from the rigid phases decay nearly to

zero during the free evolution periods in the 5-pulse

sequence (Figure 1) employed for the spin diffusion

experiments. At intermediate t (50ms, Figure 6c) a signal

with a line width of about 9 kHz is detected. We attribute

this signal to the interface between the rigid and mobile

amorphous phases. The possibility to select the magnetisa-

tion stemming from various phases can be very useful, e.g.,

in extracting parameters such as the chemical shift and the

line width of the corresponding phases. This can be, in turn,

very useful in obtaining a reliable fit of the broad-line

experiments.

Moreover, to our knowledge, this is the first time when it

is shown that the signal from the interface can be directly

selected by the proper choice of the DQ filter parameters.

This result is opening new possibilities for the investigation



of the morphology of complex polymer

systems by combining various type of

filters which select the signal steaming

from the various regions of the sample.

Domain Sizes by Proton Spin Diffusion

The heterogeneous dynamics revealed by

the DQ filter can be utilised to estimate

the domain sizes of different phases in

the PEAs. To this end the rate of proton

spin diffusion is measured following the

selection of proton magnetisation of the

rigid phase at short t in the spin diffusion

experiment and a short spin diffusion

time.

The equilibration of the magnetisation

for the sample PEA-65% after the selec-

tion of the rigid phase is depicted in

Figure 7. It can be clearly seen that at very

short spin diffusion times the magnetisa-

tion is restricted in the rigid phase. With

increase in the spin diffusion time, the

magnetisation front reaches a region

with intermediate mobility between that of the rigid

phase and the mobile amorphous phase. Only after longer

waiting times, on the order of tens of milliseconds, the

magnetisation reaches the mobile amorphous phase. These

results provide a clear evidence about the existence of an

interface between the rigid and mobile phases in the PEAs

investigated here.

The analysis of the proton spectra for each spin diffusion

time in terms of three components leads to the spin

diffusion build-up and decay curves. Based on these curves

the domain sizes can be estimated. However, an essential

step in the evaluation of reliable domain sizes is the

estimation of the spin diffusion coefficientsD. Since the rate

of the spin diffusion is related to the internuclear

distance and the molecular mobility, D has to be evaluated

for each phase. In the present work, we used the approach

proposed in ref.[51], accordingly to which

Drigid � 1

12

ffiffiffiffiffiffiffiffiffiffiffiffip

2 ln 2

rr2� �

Dn1=2 (1)

Dmobile � 1

6r2� �

aDn1=2

� �1=2(2)

where r2h i is the mean square distance between the

nearest spins, a the cut off parameter of the Lorentzian line

and Dn1=2 is the full line width at half height. A value of 4.9

A2 was estimated for r2h i in the case of the polyester

repeating unit (PEru; see Scheme 1) and a value of 4.6 A2 for



Figure 7. DQ filtered 1H NMR spectra of PEA-65% at varying spin diffusion timesmeasured using the 5-pulse sequence of Figure 1.

478

the PEA repeating unit (PEAru; see Scheme 1) based on the

known distances between the protons in the various

functional groups. The spin diffusion coefficients of the

interface were calculated as an arithmetic mean between

the spin diffusion coefficients of the rigid and the mobile

amorphous phases. The obtained spin diffusion coeffi-

cients for the samples investigated in this work are

summarised in Table 2.

Once the spin diffusion coefficients are known, the

domain sizes can be evaluated supposing that the type of

morphology adopted by each system is known. From the

SFM images (Figure 3) it is clear that all PEAs samples have a

Figure 8. Proton s65%. The symbollines represent th

Table 2. Proton spin diffusion coefficients (D) and domain sizes(d).

Polymer Rigid phase Interface Mobile

phase

D d D d D d

nm2 �ms�1 nm nm2 �ms�1 nm nm2 �ms�1 nm

PBA 0.25 7.0 0.13 0.3 0.023 2.4

PEA-10% 0.23 3.6 0.13 0.3 0.024 4.2

PEA-25% 0.23 2.4 0.13 0.75 0.030 7.4

PEA-40% 0.23 3.8 0.13 0.8 0.033 4.2

PEA-50% 0.24 4.0 0.14 0.9 0.034 3.6

PEA-65% 0.24 3.0 0.14 0.9 0.039 1.3



lamellar morphology. This is also the case

for the homopolyester PBA.[52] The

domain sizes can be then quantified by

use of general analytical solutions of the

equations describing the spin diffusion

process taking place in a lamellar mor-

phology composed of three domains of

arbitrary sizes and spin diffusion coeffi-

cients as given by Buda et al.[29] Simu-

lated and experimental time dependent

integral spin diffusion intensities for

PEA-65% are depicted in Figure 8.

The estimated domain sizes for the

PEAs are shown in Figure 9 as a function

of the amide content. As discussed

previously, the crystalline structure of

the polymers PBA and PEA-10% is differ-

ent in comparison to those PEAs with 25%

and above amide segment content. The

domain sizes corresponding to the rigid

phase first decrease on moving from PBA

to PEA-25% and then increase with

increase in concentration of amide

groups. This is in agreement with the

observed thermal properties where PEA-

25% has the lowest melting and crystallisation enthalpy

and hence, the highest amorphous content. An increment in

the enthalpies with the amount of the amide fraction is as

well observed. By comparing the polymers PEA-25% to PEA-

65%, the size of mobile component decreases and that of the

rigid component increases with increase in amide content.

The values obtained for the interface domain are smaller

compared to the sizes of the rigid and mobile amorphous

domains and these values show minor variation with the

amide content and the molecular weight of the polymers.

The ‘‘independence’’ of the interfacial thickness from the

molecular weight is in agreement with the theoretical

pin diffusion decay and build-up curves for PEA-s represent the experimental data and the solid

e simulations based on a 1D morphology.

DOI: 10.1002/macp.200900464


Figure 9. Estimated domain sizes for the rigid, interface, andmobile amorphous phases as a function of the amide contentfor the two-adjacent amide series of PEAs. The domain sizes ofPBA were included for comparison purposes.

Figure 10. Dependence of the long period of the PEAs on themolecular weight. a is the exponent of the power law relation-ship dlong / Ma

w.

predictions.[53,54] The same result was also reported for

polystyrene/polyisoprene block copolymers.[55]

To verify the reliability of the NMR measurements of the

microdomain structure, the long period dlong ¼ drigidþ2dinterface þ dmobile was plotted against the molecular

weight for the samples PEA-25% to PEA-65%. The result

depicted in Figure 10 in a double-logarithmic plot shows a

good agreement with the two-thirds power law theoreti-

cally predicted for lamellar morphologies in the strong

segregation limit.[54] The long period of the sample PEA-10%

is an exception to the above described dependence which is

attributed to its structure of the crystalline and amorphous

phases which is different from the rest of the PEA samples.

Conclusion

The morphology of segmented aliphatic PEAs, containing

two adjacent amide groups randomly placed in the

polyester backbone was successfully characterised by a

combination of DSC, SFM and solid-state proton NMR. The



investigated PEAs are semi-crystalline polymers with

lamellar morphologies for all amide fractions. Further on,

with the help of various NMR experiments it could be

shown that the morphology of these novel polymer

systems can be well described by a model accounting for

a rigid phase and a mobile amorphous phase separated by a

thin interface. Moreover, the amount of each phase and

the corresponding chain dynamics and domain sizes were

estimated and correlated with the amide content. The

obtained trend for the NMR data is in good agreement with

the measured thermal properties of the PEAs. Such

correlations of the mesoscopic and macroscopic properties

of these PEAs are important for understanding their

potential applications as biomaterials.

Received: September 2, 2009; Revised: October 19, 2009;Published online: December 9, 2009; DOI: 10.1002/macp.200900464

Keywords: morphology; poly(ester amide)s; scanning forcemicroscopy (SFM); solid-state NMR

[1] C. J. Bettinger, J. P. Bruggeman, J. T. Borenstein, R. S. Langer,Biomaterials 2008, 29, 2315.

[2] D. Yamanouchi, J. Wu, A. N. Lazar, K. Craig Kent, C.-C. Chu, B.Liu, Biomaterials 2008, 29, 3269.

[3] S. Wiegand, M. Steffen, R. Steger, R. Koch, J. Polym. Environ.1999, 7, 145.

[4] E. Grigat, R. Koch, R. Timmermann, Polym. Degrad. Stab. 1998,59, 223.

[5] G. Jokhadze, M. Machaidze, H. Panosyan, C. C. Chu, R.Katsarava, J. Biomater. Sci., Polym. Ed. 2007, 18, 411.

[6] I. Goodman, R. N. Vachon, Eur. Polym. J. 1984, 20, 529.[7] S. Yeol Lee, J. W. Park, Y. T. Yoo, S. S. Im, Polym. Degrad. Stab.

2002, 78, 63.[8] A. Alla, A. Rodrıguez-Galan, A. Martınez de llarduya, S.

Munoz-Guerra, Polymer 1997, 38, 4935.[9] US 5644020 (1997), Bayer Aktiengesellschaft; invs.: R.

Timmermann, R. Dujardin, R. Koch.[10] M. Vera, L. Franco, J. Puiggalı, J. Polym. Sci., Part A: Polym.

Chem. 2008, 46, 661.[11] T. Fey, H. Keul, H. Hocker, Macromol. Symp. 2004, 215, 307.[12] Y. Feng, D. Klee, H. Hocker, Macromol. Biosci. 2001, 1, 66.[13] C. Regano, A. Alla, A. Martınez de Ilarduya, S. Munoz-Guerra,

Macromolecules 2004, 37, 2067.[14] L. Asın, E. Armelin, J. Montane, A. Rodrıguez-Galan, J. Puiggalı,

J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 4283.[15] P. A. M. Lips, R. Broos, M. J. M. van Heeringen, P. J. Dijkstra,

J. Feijen, Polymer 2005, 46, 7823.[16] S. Bera, Z. Jedlinski, J. Polym. Sci., Part A: Polym. Chem. 1993,

31, 731.[17] H. Tetsuka, Y. Doi, H. Abe, Macromolecules 2006, 39, 2875.[18] G. E. Luckachan, C. K. S. Pillai, J. Polym. Sci., Part A: Polym.

Chem. 2006, 44, 3250.[19] N. R. Legge, G. Holden, H. E. Schroeder, ‘‘Thermoplastic Elas-

tomers’’, Hanser, New York 1987.[20] P. A. M. Lips, R. Broos, M. J. M. van Heeringen, P. J. Dijkstra,

J. Feijen, Polymer 2005, 46, 7834.



480

[21] H. R. Stapert, P. J. Dijkstra, J. Feijen, Macromol. Symp. 1998,130, 91.

[22] P. A. M. Lips, M. J. A. v. Luyn, F. Chiellini, L. A. Brouwer, I. W.Velthoen, P. J. Dijkstra, J. Feijen, J. Biomed. Mater. Res., Part A2006, 76A, 699.

[23] W. Gabrielse, H. Angad Gaur, F. C. Feyen, W. S. Veeman,Macromolecules 1994, 27, 5811.

[24] C. Hedesiu, D. E. Demco, R. Kleppinger, A. A. Buda, B. Blumich,K. Remerie, V. M. Litvinov, Polymer 2007, 48, 763.

[25] C. P. Henderson, M. C. Williams, Polymer 1985, 26, 2021.[26] C. P. Henderson, M. C. Williams, Polymer 1985, 26, 2026.[27] K. Schmidt-Rohr, H. W. Spiess, ‘‘Multidimensional Solid-State

NMR and Polymers’’, Academic Press, London 1994.[28] M. Goldman, L. Shen, Phys. Rev. 1966, 144, 321.[29] A. Buda, D. E. Demco, M. Bertmer, B. Blumich, B. Reining, H.

Keul, H. Hocker, Solid State Nucl. Magn. Reson. 2003, 24, 39.[30] A. Buda, D. E. Demco, M. Bertmer, B. Blumich, V. M. Litvinov,

J. P. Penning, J. Phys. Chem. B 2003, 107, 5357.[31] H. Yu, A. Natansohn, M. A. Singh, T. Plivelic, Macromolecules

1999, 32, 7562.[32] H. Yu, A. Natansohn, M. A. Singh, I. Torriani, Macromolecules

2001, 34, 1258.[33] A. Buda, D. E. Demco, B. Jagadeesh, B. Blumich, J. Chem. Phys.

2005, 122, 034701.[34] P. J. M. Serrano, J. P. M. van Duynhoven, R. J. Gaymans, R.

Hulst, Macromolecules 2002, 35, 8013.[35] N. S. Murthy, S. M. Aharoni, Macromolecules 1992, 25, 1177.[36] G. R. Hatfield, S. M. Aharoni, Macromolecules 1989, 22, 3807.[37] P. Garg, D. Klee, H. Keul, M. Moller, Macromol. Mater. Eng.

2009, 294, 679.



[38] D. Massiot, F. Fyon, M. Capron, I. King, S. L. Calve, B. Alonso,J.-O. Durand, B. Bujoli, Z. Gan, G. Hoatson, Magn. Reson. Chem.2002, 40, 70.

[39] P. Garg, H. Keul, D. Klee, M. Moller, Des. Monomers Polym.2009, in print.

[40] P. Garg, H. Keul, D. Klee, M. Moller, Macromol. Chem. Phys.2009, in print, DOI: 10.1002/macp.200900232.

[41] A. Aneja, G. L. Wilkes, Polymer 2003, 44, 7221.[42] R. S. McLean, B. B. Sauer, Macromolecules 1997, 30, 8314.[43] L. T. J. Korley, B. D. Pate, E. L. Thomas, P. T. Hammond, Polymer

2006, 47, 3073.[44] M. J. van der Schuur, R. J. Gaymans, Polymer 2007, 48, 1998.[45] A. Arun, R. J. Gaymans, Macromol. Chem. Phys. 2008, 209, 854.[46] D. Husken, J. Feijen, R. J. Gaymans, Macromol. Chem. Phys.

2008, 209, 525.[47] V. J. McBrierty, K. J. Parker, ‘‘Nuclear Magnetic Resonance in

Solid Polymers’’, Cambridge University Press, Cambridge1993.

[48] A. Adams, P. Garg, M. Moller, B. Blumich, unpublished.[49] B. R. Cherry, C. H. Fujimoto, C. J. Cornelius, T. M. Alam,

Macromolecules 2005, 38, 1201.[50] K. Saalwachter, Y. Thomann, A. Hasenhindl, H. Schneider,

Macromolecules 2008, 41, 9187.[51] D. E. Demco, A. Johansson, J. Tegenfeldt, Solid State Nucl.

Magn. Reson. 1995, 4, 13.[52] L. Zhao, X. Wang, L. Li, Z. Gan, Polymer 2007, 48, 6152.[53] E. Helfand, Macromolecules 1975, 8, 552.[54] E. Helfand, Z. R. Wasserman, Macromolecules 1976, 9, 879.[55] K. S. Jack, J. Wang, A. Natansohn, R. A. Register, Macromol-

ecules 1998, 31, 3282.

DOI: 10.1002/macp.200900464

morphology of novel peas containing two consecutive amide bonds randomly distributed along the...

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