Phase transition of polycarbonate in blends with liquid crystal M. Mucha Polymer Institute of Technical University, L6d~, Poland
Abstract: Phase transition temperatures of polycarbonate film consisting of micron-sized liquid crystalline droplets were investigated using differential scanning calorimetry (DSC) and thermo-optical analysis (TOA) methods. Both a decrease in (T~) and (T~) of the polycarbonate with an increase of liquid crystal (LC) content in the sample were observed, The decrease of Tx is related to the ptasfifying effect of a low molecular weight LC substance remaining soluble in the polycarbonate matrix. A fraction of the liquid crystal contained in the droplets was estimated on the basis of the Tg decrease. Key words: Polymer dispersed liquid crystal; _transition _temperatures of polycarbonate; (DSC) method; (TOA) method.
Introduction Heterogeneous composites of polymers and liquid crystals consisting of micron-sized liquid crystalline droplets in a polymer matrix show considerable promise for light- and thermo-control applications [1-5] and for permselective membranes [6-8]. Of the great variety of polymers that form transparent films, polycarbonate exhibits no thermal transition in the region of liquid crystal transition. The formation of thermoplastic polycarbonate film containing liquid crystalline droplets is achieved in a phase separation process, followed by evaporation of a solvent when the films are cast from a common solution . However, the composite films of polycarbonate with liquid crystal have shown an extra birefringence (besides that of the liquid crystal droplets), smoothing over the liquid crystal transitions in thermo-optical studies. This fact emphasizes the importance of the transition phenomena of polycarbonate in the composite film containing a liquid crystal, and prepared by casting from a solution. Using the differential scanning calorimetry (DSC) and thermo-optical analysis (TOA) methods the relationship between a liquid crystal concentration in the blends and the glass transition temperature (Tg) or the melting I:emperature (Tin) of polycarbonate was determined. 772
Some amount of liquid crystal can remain dissolved in the polycarbonate matrix where it acts as a platicizer, decreasing the glass transition temperature of the polymer. In a preparation of composite film from a common solution of polycarbonate and liquid crystal, a crystallization of polycarbonate molecules takes place. Also, some strain birefringence of polycarbonate film (perhaps occurring as a result of a difference between expansion coefficients of both components) is observed. These facts are reflected in the extra-birefringence of the composite films, which drops at Tg (relaxation of the strains) and disappears at Tm (melting of crystal phase) of polycarbonate during the heating process.
Experimental aspects 1. Samples The heterogeneous blends of the liquid crystal (LC) cholesterol nonanoate with polycarbonate (PC) (from bisphenoI A, BDH Chemical Ltd. Poole England) were prepared by casting from a solution of both components in ethylene dichloride in a proper composition of LC weight fraction of 0.01-0.5. The films of about 50/lm thickness were dried in vacuum to remove any remaining solvent. The liquid crystalline droplets had a radius not larger than 2/an.
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The DSC measurements were made using a 990(DupontCalorimeter) with a heating rate of 10 ~ The TOA studies were performed using a custom-made device described previously [9-11], which has a heating rate of 7 ~ The DSC and TOA methods permit the study of the phase transitions of LC and PC remaining in the blend. The transitions involving a variation of the heat capacity or the change of entropy can be observed in DSC studies. TOA reflects the variability of LC and PC birefringence in the course of heating or cooling processes, including the phase transition regions. Both methods complement each other . In the case of pure polycarbonate, initial strains are made by needlescratch on the film. Tg of PC estimated from DSC curves is taken as a temperature at the half-way point of heat capacity change. Tg of PC obtained from TOA curves, is taken to be a temperature at which the birefringence drops.
to 340 K is a c c o m p a n i e d w i t h an isotropic-nematic transition of LC. C u r v e 3 is a s e c o n d h e a t i n g r u n of the s a m e s a m p l e (with needle scratch). M u c h l o w e r v a l u e s of T3 a n d Tg t e m p e r a t u r e s are visible. C u r v e 7 is taken for p u r e LC a n d reveals three transition t e m p e r a t u r e s : solid-smectic (T1), smectic-cholesteric (T2), a n d cholesteric-isotropic (T3). DSC c u r v e s s h o w n in Figs. 2 a a n d 2 b are d r a w n close to Tg a n d Tm regions of PC, respectively, for s o m e s a m p l e s c o n t a i n i n g v a r i o u s a m o u n t s of LC. The c h a n g e of heat c a p a c i t y is associated w i t h Tg of
Results and discussion T O A c u r v e s ( m e a s u r i n g t r a n s m i t t a n c e vs t e m p e r a ture) 1-5 s h o w n in Fig. I are d r a w n for s o m e c h o s e n samples. The c u r v e s 1, 4, a n d 5 are initial r u n s of the films w i t h strains m a d e b y needle scratches. The t h e r m o - o p t i c a l analysis e n a b l e d us to d e t e r m i n e the p h a s e transitions: a clearing t e m p e r a t u r e of LC (T3), glass transition (Tg), a n d m e l t i n g transition (Tin) t e m p e r a t u r e s of PC. C u r v e 6 is a n initial r u n taken w i t h o u t the n e e d l e scratch. H i g h original brightness (birefringence) of the p o l y c a r b o n a t e matrix obliterates the details of the T3 a n d Tg. C u r v e 2 is t a k e n in a cooling r u n of the a n n e a l e d s a m p l e ( C u r v e 1) at 510 K (residual strains are relaxed a n d a crystal p h a s e of PC is molten). T h u s , n o birefringence at Tg a n d Tm of PC is present. In the c o u r s e of the cooling process, the increase of the t r a n s m i t t e d light intensity close
Fig. 2a. DSC curves (initial runs) taken for some PC-LC blends containing various concentrations of LC: 1) 0.01 w.f.; 2) 0.3 w.f.; 3) 0.5 w.f., 4) pure LC in the region of glass transition temperature Tg of PC. T1 = melting temperature of LC.
Fig. 1. TOA curves (transmitted light intensity vs temperature) taken for some PC-LC blends of various compositions and thermal history: 1) 0.5 w.f. LC, initial heating run (needle scratch); 2) the same sample, cooling run after annealing at 510 K; 3) 0.5 w.f., LC second heating run; 4) 0.2 w.f. LC, initial heating run (needle scratch); 5) pure PC (needle scratch); 6) 0.1 w.f. LC, initial run; 7) pure LC. T3 = clearing temperature of LC; Tg and Tm = glass transition and melting temperatures, respectively of PC.
Mucha, Phase transition of polycarbonatein blends with liquid crystal
Fig. 2b. DSC curves (initial runs) taken for some PC-LC blends containing various concentrations of LC: 1) 0.01 w.f., 2) 0.03 w.f., 3) 0.2 w.f. in the region of melting temperature Tm of PC. PC. In addition to the Tg, two endothermic peaks are observed (first order phase transitions). The one at lower temperature (- 345 K) is a residual peak due to LC meIting -T1 (see curve 4; the broad peak involves T~, T2, and T3 transitions); the curve one at higher temperature corresponds to the melting =Tm of PC matrix (~ 490 K). No peaks are recognized due to T2 and T3 of LC consisting of PC (low heat of the transitions). The effect of LC content (weight fraction Ca) on Tx and T~ of PC is presented in Fig. 3. In general, an T~,K
9 increase in LC concentration reduces both Tx and Tm of PC. Tg values determined in the second heating run TOA (curve 2) are m u c h lower than Tx obtained in the first heating run TOA (curve 1); this is due to the great liquid crystal solubility in the polycarbonate matrix at higher temperature. Also, the relaxation of the strains and the melting of crystal phase of PC perhaps affects the observed decrease of Tg. The reduction of the glass transition temperature of polycarbonate indicates that an appreciable a m o u n t of LC can remain dissolved in the matrix; the rest is located in the microdroplets. It is well k n o w n [12, 13] that the addition of small molecules to a polymer can reduce the glass transition temperature, i.e., act as a plasticizer. A simple Fox equation  based on the theory of Gordon and Taylor  relates the glass transition temperature Tg of a plasticizer sample to the weight fractions C1 and Ca, and Txl and Tx2 of the pure polymer and the pure plasticizer: 1 Zg
C1 C2 + Zg1 Zg2"
The Tg value determined for glass-forming liquid crystals seems to be on the order of 200-230 K. We haven taken Tg2 = 220 K for cholesterol nonanoate, and T~I = 430 K for polycarbonate. Both plots 1/Tg vs C~ (curves I and 2) in Fig. 4 are not linear; Eq. (1) does not exactly fit the presented system. (Line 4 is directly d r a w n according to Eq. (1).) A w e i g h t fraction of LC dissolved in PC is m u c h lower than that used
4aol Tg,K ,
• 3s 1/K
41E ~ 400 390
0'.2 013 0'.4 o.'s C~
Fig. 3. Plots Tg and Tr~ of PC vs weight fraction LC (C2) in the samples derived from: 1) initialheating run TOA;2) secondheating run TOA. x = results of DSC studies.
0.5 . . . . C~
Fig. 4. Plots of 1/Tg vs weight fraction of PC (C1): 1) and 2) Tg determined from initial and second heating runs TOA; 3 straight line dependence in low concenl~ationof LC (second heating run); 4) drawn according to the Fox equation (1).
Colloid and Polymer Science, Vol. 269. No. 1 (1991)
LC solubility in the PC matrix (curve 2) during annealing at a higher temperature, and with decreasing of the amount of LC in the sample (Ca). Decreasing of K points to the unadvantageous reduction of LC efficiency in the blends with PC. Moreover, the purity of LC in the droplets remains higher the larger the concentration of LC in the blends. Some amount of the polymer or a solvent is retained in the LC droplets as an impurity. This fact is due to the small decrease in the clearing (T3) and melting (T~) temperatures of LC contained in the blends with PC.
0.6 0.4 0.2 0
. . . .
Fig. 5. Fraction K of LC contained in the droplets determined from Tg following: 1) initial heating run; 2) second heating run drawn vs weight fraction C1 of PC in the blends.
(C2) in the calculation. The fraction K of a liquid crystal contained in the microdroplets is of interest because of efficiency of the LC-PC blends. It is possible to estimate K from the glass transition temperature of the matrix by modification of Eq. (1) using the following expression for K: K= 1-
1 - Tg
C'1 + C'2 = 1
Zg2 . (Tg I - Zg) .__1 (Zg I - Zg2) Zg C2
where C1' is the weight fraction of PC in the matrix, C2' is the weight fraction of LC dissolved in the matrix, and C2 is the weight fraction of LC in the sample. Values of K derived from Eq. (5) are plotted in Fig. 5 vs C1. We found that K decreases with increasing
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Author's address: Dr. M. Mucha Polymer Institute of Technical University Zwirki 36 90-924 L6d~, Poland