Morphology oftrans-1, 4-polyisoprene crystallized in thin films

Abstract

The development of melt-grown crystalline morphology in thin films of trans-1,4-polyisoprene (TPI) was studied by optical and electron microscopy. The crystalline morphologies observed are explained in terms of thin chainfolded lamellar crystals which would grow (if unrestricted) to a diamond shape. The electron diffraction patterns obtained can be indexed using the unit cells proposed by Fisher [3]. The preferred growth faces for the low melting form crystals (LMF) are the (120) planes and probably the (110) planes for the high melting form crystals (HMF). LMF crystals are exclusive to LMF spherulites and HMF crystals are exclusive to HMF spherulites. At large supercoolings both LMF and HMF spherulites nucleate as bundles of lamellar crystals and grow by extensive, twisting, branching and spawning. LMF spherulites grown at small supercoolings develop as hedrites/axialites, or as splayed groups of large crystals differing in orientation with respect to the electron beam. The frequency of twisting, branching and spawning being minimal at these supercoolings. Row nucleated structures are observed in strained films. They consist of long wavy backbone crystals (5 to 25 nm thick) lying in the direction of strain, overgrown by lamellar crystals (5 to 10 nm thick) oriented at right angles to the strain direction. At some temperatures both LMF and HMF crystals nucleate in the same row. The morphologies observed in thin films are compared with and discussed in terms of the observations made of bulk-grown and solution-grown crystals.