Phase Separation and Viscoelastic Behavior of Semicompatible Polymer Blends: Poly(vinylidene fluoride)/Poly(methyl methacrylate) System

Abstract

ABSTRACT:Thermal and dynamic mechanical analyses of variously processed blends of poly(vinylidene fluoride) (PVF2) and poly(methyl methacrylate) (PMMA) were carried out to elucidate the effects of lower-critical-solution-temperature (LCST) behavior reported earlier by Paul on the morphologies and properties of this system. The two components are miscible in the blends with PVF2 content 4>w < 0.6. Such blends exhibit single glass transition at T;s, which follow the Gordon-Taylor scheme. On the other hand, the blends with c/Jw > 0.6, prepared by melt-mixing and annealing at 200oc for a sufficiently long period of time, exhibit two T.'s and two Tm's, implying a four-phase morphology. In such blends an LCST-type phase separation has taken place at a temperature as low as 200oc to give an amorphous mixture with 4>w 0.6 and a nearly pure PVF2 melt. Quenching of such melts leads to a four-phase morphology composed of a mixed amorphous phase with T. 55°C, an imperfect crystalline phase grown out from the mixture and hence having a low Tm, and a nearly pure semicrystalline PVF2 domains similar to form II crystals with -40oC and a higher Polymer blends of poly(vinylidene fluoride) (PVF2) and poly(methyl methacrylate) (PMMA) have been the subject of several recent studies. 1 -9 The general conclusion is that these components are miscible in the amorphous state, i.e., above the melting point of PVF2 l70°C). Below Tm the PVF 2 crystallizes from the melt, especially with high PVF2 content, ¢w, if the blend is kept above its glass-transition temperature, Tg. The blend forms a two-phase system consisting of a PVF 2 crystalline phase and a mixed amorphous phase. In short, this system is a typical example of a semicompatible polymer blend composed of a semicrystalline polymer and an amorphous polymer, in which phase t Present address: Research and Development Department, Bridges/one Tire Co., 2800-1 Ogawahigashimachi, Kodaira 187, Japan. separation occurs only by crystallization of the former. However, Paul et a/. 6 pointed out that this system exhibits lower-critical-solution-temperature (LCST) behavior with cloud points, TP, in a temperature range of about 330°C for blends with high ¢w· The TP are far above the ceiling temperature for PMMA Such an LCST behavior, if it really exists, might bring about additional complications to the phase-separation behavior and physical properties of PVF2/PMMA blends. If fact, there are certain discrepancies between reported Tg versus composition data for melt-mixed blends 2 • 5 and for solvent-cast blends 3 of this system. Thus, we have attempted to resolve these problems by comparing the thermal and dynamic mechanical behavior of melt blends with that of solvent-cast blends of this system. The results obtained are reported in this paper.

D.m.a. and d.s.c. investigations of the β transition of poly(vinylidene fluoride)

“…Recently, Martines-Salazar et al [13][14] have reported that the glass transition temperature-composition relationship for the completely amorphous PVDF/PMMA blends could be fitted by the Gordon-Taylor equation [16]. This result is in accord with previous observations by Hirata and Kotaka [6], at least until the PVDF content does not exceed 60wt%. Indeed, beyond this composition, PVDF starts to crystallize and discrepancy is reported to occur.…”

“…The transition behaviour of semicrystalline poly(vinyli-dene fluoride) (PVDF) has been extensively studied [1][2][3][4][5][6][7][8][9][10]. In addition to melting, PVDF usually exhibits at least two transitions that have been assigned to the α-transition and the β-transition, respectively, as the temperature is increased.…”

“…In addition to melting, PVDF usually exhibits at least two transitions that have been assigned to the α-transition and the β-transition, respectively, as the temperature is increased. It is accepted that the α-transition stems from molecular motions in the PVDF crystalline region [6][7]. However, two different mechanisms have been proposed for the β-transition.…”

“…The β-transition is proposed to be characteristic of the crystal-amorphous interphase. The second model relies upon the coexistence of mixed amorphous phase, imperfect crystalline region, and nearly pure semicrystal-line PVDF domains consisting of crystalline and amorphous phases [6]. This model rests on d.m.a, and d.s.c.…”

“…This model rests on d.m.a, and d.s.c. The β-transition is then assigned to the PVDF amorphous region [6]. Recently, Martines-Salazar et al [13][14] have reported that the glass transition temperature-composition relationship for the completely amorphous PVDF/PMMA blends could be fitted by the Gordon-Taylor equation [16].…”