Effect of substrate on the crystallization of polyvinylidene flouride/poly(methylmethacrylate) polyblend from solution

Abstract

Blends of Polyvinylidene flouride (PVDF) and Poly(methylmethacrylate) (PMMA) are known to be compatible over a wide range of composition [1-5] in the amorphous state but PVDF crystallizes in the mixture when the weight fraction of PVDF is more than 0.5. PVDF exhibits at least four crystalline phases (i.e. c~,/~, 7 and 5). Its piezo and pyre-properties are due to polar /?-form which has a planar trans-structure.//-phase is not usually produced by crystallization from the melt, but is normally obtained by mechanical deformation of e-form [6-7] PVDF which is non-polar./?-form may also be grown from solution under special conditions [8]. Growth from solutions such as dimethyl formamide or dimethyl sulphoxide or dimethylacetamide (DMA) usually leads to 7-form [3]. In this letter, preliminary results on the crystallization of different forms of PVDF in the 60:40 weight ratio of the PVDF/PMMA polyblend grown from DMA solution onto different substrates are described along with some studies by infrared spectroscopy (IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). PVDF material in powder form was obtained from Janasen Chimica, Belgium. PMMA material (low molecular weight) was obtained from BDH, England. Polyblend was prepared by dissolving the two polymers in 60:40 weight proportion in DMA at 60 ° C. Thin films of the polyblend (MF0.6) were prepared by casting this solution on glass, aluminium and stainless steel substrates at 60°C in an air-oven. IR spectra of film samples were obtained on a PerkinElmer spectrophotometer (model 580 B). XRD experiments were done on a Rigaku Geigerflex diffractometer in the reflection mode using CuK~ radiation at a scanning speed of 3 degmin -I. SEM micrographs were taken on the SEM Cambridge Stereoscan model $4-10. X-ray diffractograms of the film samples are shown in Fig. 1. They reveal that films grown on steel and glass substrates lead mainly to the eor 7-polymorph. The d-spacing, for the Bragg angle 20 = 20.24 and 20.32 °, for steel and glass substrate samples, are found to be 0.438 nm and 0.436 nm respectively. These values correspond to reflections from (1 10) and (021) planes of phase III [9] (7-phase) crystallites and for phase II PVDF (e-form) i.e. these reflections do not uniquely determine the phase of PVDF. Intensity of the peak for the glass substrate was however found to be approximately double than that of the stainless steel sample. For the aluminium substrate a peak was found at 20 = 19.9 ° which is attributed to the presence of e-form [10]. IR spectra of films are shown in Fig. 2. It is clear from the figure that the intensity of IR-bands corresponding to e-PVDF [11] increases for the aluminium substrate (e.g. peaks at 615 and 765cm -~ and a shoulder appearing at 796 cm-~). Characteristic 7-phase peaks appear in all the sample (430cm-1). Intensity of certain/~-peaks eg. at 510 and 840 cm-J is also enhanced in these samples as compared to the IR-bands observed in PVDF films [12] at these frequencies. This may be due to the presence of PMMA in the polyblend which has been found to favour the e --* /~ transition. From the analysis of IR and XRD results on blend samples grown on different substrates, it is clear that the aluminium substrate favours e-polymorphism while the glass and steel substrates favour more 7but almost an equal growth of /~and a-phases. The surface topography of the cast films can be more clearly seen in the SEM photographs of Fig. 3. Films cast on steel and glass show distinct spherullites co-existing with a hill-like (fibralliar) structure while films cast on aluminium show a very different feature.