Thermal Pressure Coefficient of a Polyhedral OligomericSilsesquioxane (POSS)-Reinforced Epoxy Resin

K. Z. Win, Taskin Karim, Qingxiu Li, Sindee L. Simon, Gregory B. McKenna

Department of Chemical Engineering, Texas Tech University, Lubbock, Texas 79409

Received 17 February 2009; accepted 27 September 2009DOI 10.1002/app.31502Published online 23 November 2009 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The thermal pressure coefficients of a neat,unfilled, epoxy resin and a 10 wt % POSS (polyhedral oligo-meric silsesquioxane)-filled epoxy nanocomposite have beenmeasured using a thick-walled tube method. It is found thatjust below the glass transition temperature the thermal pres-sure coefficient is � 20% smaller for the polymer compositecontaining 10% POSS than for the neat, unfilled resin. The

thermal expansion coefficient and thermal pressure coeffi-cient of the uncured POSS itself are also reported. VC 2009Wiley Periodicals, Inc. J Appl Polym Sci 116: 142–146, 2010

Key words: nanocomposites; residual stress; thermalpressure coefficient; POSS; thermoset; epoxy; polymer;high-pressure vessel

INTRODUCTION

Widespread applications of thermosetting polymerresins in composite materials call for a better under-standing of their thermal and mechanical properties.One shortcoming of these resins is that residualstresses can arise in the composite because of thedifferences in volume changes between the resin andthe reinforcing fibers or between the composite andexternal constraints, such as mold walls, when theyare subjected to temperature changes. These residualstresses may cause early failure or sub-par perform-ance of the composite system. In typical applica-tions, the resin can be constrained from shrinking inone, two, or three dimensions, and these constraintslead to the build-up of potentially large stressesin the resin. The magnitude of the two-D stressbuild-up1,2 is related to the thermal stress coefficientaLE (aL is the linear coefficient of thermal expansionand E is the Young’s modulus), and the three-Dstress build-up3–5 is due to the thermal pressurecoefficient c (c ¼ aK, where a is the volumetric coef-ficient of thermal expansion and K is the bulk modu-lus.) Recently, Li et al.6 investigated the thermalstress coefficient (as the product of shear modulus Gand a) in a POSS/epoxy nanocomposite system, andthis note expands on that work and presents the

results of an investigation of the impact of adding10 wt % polyhedral oligomeric silsesquioxane(POSS) on the thermal pressure coefficient aK ¼ c ofthe resulting POSS/epoxy nanocomposite system.In some instances,4,7,8 a and K have been consid-

ered to be nearly reciprocal (within a constant) evengoing through the glass transition. In such a case,c � constant. If this is true in general, strategies tomitigate thermal stresses by decreasing a of a systemmay be thwarted if K increases by a magnitude simi-lar to the decrease in a. However, in recent work,Alcoutlabi et al.9 have compiled literature data thatshow that c in rubbery and glassy states of a rangeof polymers may differ significantly and that itvaries significantly among polymer systems. Thissuggests that in addition to changing chemical struc-ture of polymer resins, there may be strategies avail-able to modify polymers through judiciously chosenreinforcing particles to reduce the thermal pressurecoefficient and to, therefore, reduce thermallyinduced residual stresses.The motivation for this investigation of the

POSS/epoxy system is that the unique cage-likestructure10–12 of POSS might be expected to exhibitdifferent reinforcing effects for thermal expansioncoefficient and mechanical moduli as the thermalexpansion coefficients of the cage-like POSS moietydepend mainly on the bond energies, whereas themoduli of the POSS depend on the flexing/bendingof the bonds in the cage. In the organic hybrid POSSused here, there are also contributions from theinteratomic and intermolecular interactions. In addi-tion, despite its unique cage-like structure, the prop-erties of POSS have not been as extensively studiedas, e.g. the cage-like structured Bucky-balls.13 We

Correspondence to: G. B. McKenna (greg.mckenna@ttu.edu).

Contract grant sponsor: National Aeronautics and SpaceAdministration (NASA); contract grant number:NNX07AD44A.

Journal of Applied Polymer Science, Vol. 116, 142–146 (2010)VC 2009 Wiley Periodicals, Inc.

base this work on the hypothesis that using POSS inan epoxy resin provides the opportunity to mitigatethe bulk thermal stresses by reducing a without aproportional change in K. The work is complemen-tary to the recent work by Li et al.6 from these labo-ratories that the thermal stress coefficient aL G canbe reduced (above room temperature) by addition ofPOSS to an epoxy, consistent with this hypothesisthat the thermal pressure coefficient c ¼ aK can alsobe changed. This report provides results of the mea-surement of the thermal pressure coefficient c ¼ aKfor a neat epoxy resin and the resin with 10 weight% POSS added.

MATERIALS AND METHODS

The epoxy resin used in this study was diglycidylether of bisphenol A (DGEBA) (DER332 Dow Chem-ical), the POSS was polyepoxyglycidyl silsesquiox-ane (EP0409 Hybrid Plastics), and the curing agentwas tetrafunctional aromatic diamine (Versalink 740AirProducts). The structures of the POSS, the epoxy,and the curing agent are shown in Figure 1. Epoxy,aromatic diamine, and POSS (or epoxy and diamine)were mixed in a stoichiometric ratio of functionalgroups and stirred at 100�C for at least 30 min untilthe mixture was homogeneous. Dissolved gas wasremoved before cure by pumping in vacuum at

room temperature for 30 min and then at 100�C for� 1 h until air bubbles were no longer observed.(Based on the results of Wisanrakkit and Gillham14

for the epoxy resin and from Merzlyakov et al.,15

this would result in partial reaction but would notresult in gelation of the system).Measurement of the thermal pressure coefficient

c was performed using the thick-walled tubemethod of Merzlyakov et al.16 developed in thelaboratories of Texas Tech University. Here, webriefly describe the method and modifications wehave made to the original procedure for thisstudy. The thick-walled tube in this method is astainless steel pressure nipple from High PressureEquipment Co. (60-HM4-2.75–316). On the outerwall of the tube a strain gage is attached to mea-sure the stresses of the sample inside the tube. Asit was already shown16 that stresses measuredusing axial and hoop gages are the same, here, weused only the hoop gage for our measurementbecause of its greater sensitivity. In addition to theFSM-series gages (Vishay) mentioned in Ref. 16,we also used general purpose strain gages fromVishay with equal success although the FSM gagesare more robust and thus last longer. The straingage is placed in one arm of a Wheatstone bridgewith three other fixed-value resistors at the otherarms. Here, we replaced the trim-pot from Ref. 16with a resistor (S102C Vishay) whose value(350 X) matches very closely to that of the straingage. As a result, the bridge condition is slightlymore unbalanced at the outset (i.e. at room tem-perature, zero pressure), but this is acceptable aswe are ultimately measuring how much the bridgegets out of balance from this baseline; whether thebaseline is about 0.1 lV (balanced with a trim-pot)or about 10 lV (with 350 X fixed-value resistor)does not affect the measured stress. The pressureresponse of the strain-gaged tube at room tempera-ture was calibrated by pressurizing the tube withvacuum-pump oil and recording the bridge voltageas a function of pressure. Similarly, the tempera-ture response was calibrated by recording the volt-age response as a function of temperature for theempty tube in a temperature-programmable oven.We remark that, as shown below, the presentmeasurements of c for the neat resin are some-what higher than those obtained by Merzlyakovet al.16 The reasons for this seem to be related tothe details of the experimental procedures used.However, all experiments reported here are for asingle thick walled tube device and are consideredto be valid within the set of experiments per-formed here. Reasons for the differences betweensets of experiments run on different tubes and bydifferent investigators remain to be established.

Figure 1 (a) Structure of POSS molecule used in presentstudy; (b) diglycidyl ether of bisphenol A (DGEBA);(c) trimethylene glycol di-paminobenzoate. (After Ref. 6).[Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

THERMAL PRESSURE COEFFICIENT POSS-REINFORCED EPOXY RESIN 143

Journal of Applied Polymer Science DOI 10.1002/app

RESULTS AND DISCUSSION

A typical test is run by first loading the uncuredsample into the strain-gaged tube at 100�C in a vac-uum oven (under vacuum) using a shortened-pipettewith one end closed. After filling the tube with thesample, the other end of the tube is also closed.Next, a thermocouple is attached to the pressurenipple, which is then put in an oven, and the stressand temperature are recorded as the oven tempera-ture is changed according to a preset temperatureprofile. To compare our results with the previousdata of Merzlyakov et al.,16 we followed the curingschedule reported in that study: the filled tube isheated to the curing temperature at 1 K/min in anoven and then held at that temperature for 10 h. Thedevice is then allowed to cool back to room tempera-ture. The cooling rate is below 1 K/min and the rateslows as the oven temperature approaches roomtemperature. The results for the neat resin are shownin Figure 2. The zero value of the pressure at roomtemperature is defined for each run by shifting thedata vertically. This does not affect capp as it isdetermined from the slope of the pressure–tempera-ture plot.

As the sample is heated from room temperature, ahydrostatic compression (pressure) develops in theresin due to the fact that it cannot expand due to theconstraining tube walls. At � 100�C, the resin beginsto cure, which results in a decrease in volume asvan der Waals bonds are converted to covalentbonds. In Figure 2, this manifests itself in a decreasein the initially linear P vs. T profile. As temperaturecontinues to increase, the reaction rate accelerates,and at � 120�C, the rate of shrinkage due to cureovertakes the rate of thermal expansion and thecompressive pressure in the resin decreases(becomes less negative). At the end of the 10-hour

cure at 160�C, the sample is cooled to room tempera-ture, and the pressure decreases and becomes ‘‘posi-tive’’ as the sample goes into hydrostatic tension.One sees a rapid drop in the hydrostatic tension at� 90�C as the sample debonds and/or breaks fromthe transducer (tube) wall. For the temperaturerange of 140–155�C, the slope of the cooling curvegives the apparent thermal pressure coefficient ofthe cured resin, capp as 0.76 MPa/K; this is greaterthan the result of 0.64 MPa/K obtained earlier forthe same material.16 The discrepancy may be due toour use of a different pressure vessel, strain gageattachment, and/or minor modification of the elec-tronics. However, all experiments in this study wereperformed using a single thick-walled tube deviceand are considered to be consistent within this set ofexperiments.In Figure 3, we show the cooling response after

curing for the epoxy þ 10% by weight POSS nano-composite. The two traces correspond to two differ-ent samples. From the figure, we see that the hydro-static tension develops nearly identically for the twosamples until the cured samples break from thetransducer walls at � 17 and 21 MPa, respectively.For the temperature range of 140–155�C, we findthat the apparent thermal pressure coefficient capp is0.60 and 0.59 MPa/K for the two different runs.Hence, the nanocomposite has a lower thermal pres-sure coefficient than the neat resin, consistent withthe original hypothesis of the work and consistentwith the prior work by Li et al.6 for the thermalstress coefficient.Because the type of POSS we use (EP0409) is a vis-

cous liquid at ambient conditions, it was also possi-ble to determine the thermal pressure coefficient ofthe POSS. Figure 4 shows two cooling curves for thePOSS; capp is found to be 1.05 and 1.02 MPa/K, forthe two tests in the temperature range 140–155�Cthat is in the liquid state. We also measured the

Figure 2 Pressure vs. temperature during heat-hold-coolcycle for the neat epoxy resin.

Figure 3 Cooling curves after curing the epoxy and POSSmixture. Different lines represent separate samples.

144 WIN ET AL.

Journal of Applied Polymer Science DOI 10.1002/app

nominal value of a for the liquid POSS itself for thetemperature range 25–150�C to be 2.6–3.8 � 10�4/K.This value is expected to be between that of silicafor which it is very small (1–10 � 10�6/K) and theorganic group for which it would be larger. Ourmeasured value is much higher than that of silicaindicating that the thermal behavior of the organi-cally modified POSS is dominated by its organiccomponent. Li et al.6 have measured a for neat andPOSS-filled resin at 150�C to be 4.2 � 10�4/K and4.0 � 10�4/K, respectively.

The values of the thermal pressure coefficientsmeasured from the slope of the pressure vs. temper-ature plots in Figures 2–4 are only apparent valuesbecause the confining tube both has a mechanicalcompliance and its own coefficient of thermal expan-sion, i.e., the measurements are not performed underideal isochoric conditions. Merzlyakov et al.16

derived an equation to obtain the actual c from theapparent values (capp) determined from the meas-urements of pressure and temperature for a filledand instrumented tube

c ¼ cappð1þ K=KTÞ=ð1� aT=aÞ (1)

Equation (1) involves two dimensionless numbers:aT/a and K/KT where the subscript T indicatesthe value for the steel tube. We take16 aT ¼ 4.86 �10�5/K and KT ¼ 55.6 GPa. We estimate K by usingcapp and the value of a cited above and find that K¼ 1.79 GPa and 1.50 GPa for the neat resin and thePOSS-filled resin, respectively. It is interestingthat the value of the bulk modulus obtained forthe POSS filled resin is � 26% lower than that of theneat resin, consistent with the hypothesis that thecage structure might flex somewhat in a mechanical

deformation. This comment is made with caution,however, because the complex structure of theorganically modified POSS may also be a majorcontributor to this behavior and because the POSSparticles are chemically bonded through the epox-ide linkages to the polymer network and the nano-composite is not a simple mixture of the particlesand the matrix. Using these values for K in eq (1),we estimate that c ¼ 0.88 MPa/K for the neatresin and 0.70 and 0.69 MPa/K for the POSS-filledresin (Table 1).Although we have measured only a relatively

small change in c, such a change may still be veryimportant. McKenna and Penn17,18 and Crissmanand McKenna,19 for example, found that failurelifetime of polymers follows a very strong powerlaw in the applied stress. For example, assuming19

tfailure ! r�19 where r is the residual stress andthe reduction in stress on the order of 20% asfound above for c, we find that tfailure increases bynearly a factor of 70.

FUTURE WORK

The present thick-walled tube method requiressomewhat more correction for the tube’s thermalexpansion and mechanical compliance than is desir-able. As seen in eq (1), the dominant correction (fora sample with comparable properties to the resinstudied here) comes in the thermal expansion coeffi-cient term that contributes about 25% whereas thestructural stiffness or compliance term contributesless than 10%. Increasing the tube wall thickness,while feasible, reduces the sensitivity of the devicethat already required the use of a lock-in amplifierto measure the small strains in the tube wall.16 Onthe other hand, using a material like Invar that has avery low thermal expansion coefficient combinedwith a structural stiffness comparable to the steelused in the current device configuration offers theopportunity to improve the thick walled tube methodof thermal pressure coefficient determination.

Figure 4 Pressure-temperature plot from cooling of POSS(EP 0409). Two lines representing two samples are offsetvertically for clarity.

TABLE IThermal Pressure Coefficients of POSS-Filled Resin and

Its Constituents from 140–155�C. To Determine theActual Thermal Pressure Coefficient c of Pure Poss WeUse eq (1), Our Measured Value of a 5 3.2 3 1024/K and

the Bulk Modulus K 5 7.5 GPa from MolecularSimulations20 for a Similar POSS

Material capp (MPa/K) c (MPa/K)

Neat epoxy resin 0.76 0.8810 wt % POSS/resin 0.59, 0.60 0.69, 0.70Liquid POSS 1.02, 1.05 1.36, 1.41

THERMAL PRESSURE COEFFICIENT POSS-REINFORCED EPOXY RESIN 145

Journal of Applied Polymer Science DOI 10.1002/app

CONCLUSIONS

This work has shown that the glassy state thermalpressure coefficient c is � 20% smaller for a 10 wt %POSS-filled epoxy resin nanocomposite compared tothe value of c for the neat resin over the temperaturerange 140–155�C, i.e., just below the glass transitiontemperature. The value of c for the POSS alone hasalso been determined, and we report estimates ofthe values of the bulk modulus for the epoxy neatresin and for the POSS-filled system as well.

The authors thank M. Merzlyakov for useful suggestionswith the experimental set-up.

References

1. Brahatheeswaran, C.; Gupta, V. B. Polymer 1993, 34, 289.2. Lange, J.; Toll, S.; Manson, J. A.; Hult, A. Polymer 1995, 36,

3135.3. Plepys, A. R.; Farris, R. J. Polymer 1990, 31, 1932.4. Adolf, D.; Chambers, R. Polymer 1997, 38, 5481.5. Prasatya, P.; Mckenna, G. B.; Simon, S. L. J Compos Mater

2001, 35, 826.

6. Li, Q.; Hutcheson, S. A.; Mckenna, G. B.; Simon, S. L. J PolymSci Part B: Polym Phys 2008, 46, 2719.

7. Adolf, D.; Martin, J. E.; Chambers, R. S.; Burchett, S. N.;Guess, T. R. J Mater Res 1998, 13, 530.

8. Adolf, D.; Martin, J. E. J Commun Mater 1996, 30, 13.

9. Alcoutlabi, M.; McKenna, G. B.; Simon, S. L. J Appl Polym Sci2003, 88, 227.

10. Hybrid Plastics 2006 Catalogue. Available at http://www.hybridplastics.com/catalog.htm.

11. Li, G. Z.; Wang, L. C.; Ni, H. L.; Pittman, C. U. J Inorg Orga-nomet Polym 2001, 11, 123.

12. Scott, D. W. J Am Chem Soc 1946, 68, 356.

13. Kroto, H. W.; Heath, J. R.; O’brien, S. C.; Curl, R. F.; Smalley,R. E. Nature 1985, 318, 162.

14. Wisanrakkit, G.; Gillham, J. K. J Appl Polym Sci 1990, 41,2885.

15. Merzlyakov, M.; Mckenna, G. B.; Simon, S. L. Compos A 2006,37, 585.

16. Merzlyakov, M.; Simon, S. L.; Mckenna, G. B. Rev Sci Ins2005, 76, 063904.

17. McKenna, G. B.; Penn, R. W. Polymer 1980, 21, 213.

18. McKenna, G. B.; Penn, R. W. J Biomed Mater Res 1980, 14,689.

19. Crissman, J. M.; McKenna, G. B. J Polym Sci Part B: PolymPhys 1987, 25, 1667.

20. Capaldi, F. M.; Boyce, M. C.; Rutledge, G. C. J Chem Phys2006, 124, 214709.

146 WIN ET AL.

Journal of Applied Polymer Science DOI 10.1002/app