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STIINTA MATERIALELORCURS 11POLIMERI: STRUCTURA, TIPURI STRUCTURALE ; POLIMERI TERMOPLASTI SI TERMORIGIZI,
ELASTOMERI
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POLIMERIDef.: Materiale organice macromoleculare constituite din lanturi lungi (M =
104 – 106 Da) - C – C – (legatura covalenta) la care se asociaza si altiatomi (H, O, N, F, ...)
Polimerii sunt alcatuiti din unitati structurale care se repeta
Ex. Polietilena (PE) - (CH2 – CH2)n - Mu.s. = 28Da
Polimetacrilat de metil (PMMA) Mu.s. = 100Da
Homopolimeri / Copolimeri aleatori A-B-A-B-B-B-A-A-alternanti A-B-A-B-A-B-A-B-in blocuri A-A-A-A-B-B-B-B-
-
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nCOOCH
C
CH
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POLIMERI - STRUCTURAPolimer cu catena liniara - U – U – U – U – U – U - ....
U = unitate structurala
Polimer ramificat (cu catene laterale)
Polimer reticulat (cu legaturi transversale)
UUU
UUUUUUU
UUU
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POLIMERI - STRUCTURAPolimeri Amorfi catena liniara / ramificata
Semicristalini catena reticulata –prin ordonare in siruri semiparalele
Grad de cristalinitate = raportul fractie ordonata / cant. totala
Cristalizare – la cald, sub sarcini mecanice, chimic
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POLIMERI - STRUCTURA
Structura 3D semicristalina in nylon 6,6 prin legatura de H
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POLIMERI - STRUCTURA
Polimer amorf Polimer semicristalin Sferulita
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POLIMERI - STRUCTURA
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POLIMERI - STRUCTURA
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POLIMERIMasa macromoleculei: M = n x Mu.s. , n = grad de polimerizare(n < 100 pt. oligomeri)
Mn – masa moleculară medie numerică; Mw – masa moleculară medie masică
Polimerii industriali: Mn = 25,000 – 100,000Da; Mw = 50,000 – 300,000Da
i = Mw / Mn – indice de polidispersitate;
Polimerii de sinteză – amestecuri de macromolecule cu grade diferite de polimerizarePolimeri comerciali: i = 1.5 – 5; polimeri naturali: i ~ 1 (general)
åå=
i
iin N
MNM å
å=ii
iiw MN
MNM
2
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POLIMERI
Masa moleculară infuenţează major proprietăţile polimerului:
Exemplu – polietilena
PE-HD (0.945-0.960 g/cm3): R = 22-30 MPa; A = 500-800; tmax = 70-80ºC
PE-LD (0.915-0.924 g/cm3): R = 9-11 MPa; A = 500-650; tmax = 60ºC
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POLIMERI
Cele 4 legaturi simple ale atomului de carbon definesc un tetraedru regulat
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POLIMERI
Polistiren
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POLIMERI
Legatura C-C se poate roti in jurul axei proprii
>>> MOBILITATE MARE (minimizarea energiei)
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POLIMERISinteza polimerilor – pornind de la monomeri
(etilena, propilena, benzen, ...); se rup legaturi duble si se formeaza noi legaturi simple
Poliaditie - PE, PP, PVC, PS, PUR, ...
Policondensare – se elimina o molecula mica (H2O, HCl) PA (Nylon), PF, ...
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POLIMERITipuri de polimeri
a. Termoplasti – trec in stare vascoasa la cald (T > Tc)catene liniare / lanturi laterale
b. Elastomeri (cauciucuri) – legaturi transversale slabe
cauciuciri polisulfurice (tiocauciucuri) prin vulcanizare → legaturi tranversale de sulf
continutul de S (3 – 25%) → rigiditate / duritate[peste 50% - ebonita]
Vulcanizarea cu S a cauciucului (natural)
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POLIMERIcauciucuri siliconice
Cauciucul siliconic
Interval de temperaturi de lucru: -120ºC – 300ºCDuritate Shore A: 10 - 90
Poliizopren – sintetic, similar cauciucului natural;Polietilena clorurata – rezista la cald in hidrocarburi lichide;Polibutadienic / polibutadienstirenic – rezistenta buna sub 0ºC
>>> in anvelope;Poliacrilic – rezistent in produse petroliere;
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POLIMERI
Curba termomecanica pentru polimeri amorfi; Tg – temperatura tranzitiei vitroase; Tf – temperatura de curgere; Tg > Tambiant → termoplasti; Tg < Tambiant → elastomeri
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POLIMERIc. Termorigizi (duromeri) – polimeri la care intarirea (la cald - >200º; prin reactie – epoxi; prin iradiere) este ireversibila
Prin incalzire se distruge macromolecula, nu apare inmuiereaAu legaturi transversale tari
Polimerii epoxidici
Polimerizarea prin reactie intre o rasina epoxidica (avand grupuri epoxid la capete) si un intaritor (poliamina)
Grupul epoxid Rasina epoxidica
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Intrebari de autoevaluare1. Ce sunt polimerii?2. De cate tipuri structurale sunt polimerii?3. Care sunt modalitatile de exprimare a masei unui polimer? Prin ce
difera?4. Ce este indicele de polidispersitate?5. Care este diferenta intre un homopolimer si un copolimer?6. Cum se poate modifica gradul de cristalinitate a unui polimer?7. Cum influenteaza gradul de tacticitate proprietatile unui polimer?8. Care este motivul mobilitatii in catena?9. Care sunt metodele de sinteza a polimerilor? Care sunt
mecanismele de realizare a acestora?10. Ce este vulcanizarea?11. Care sunt avantajele cauciucurilor siliconice?12. Prin ce difera, din punctul de vedere termomecanic, polimerii
termoplasti de elastomeri?13. Cum are loc polimerizarea polimerilor epoxidici?