Inclusion of perfluoromethyl groups in the crystals of copolymers of tetrafluoroethylene and hexafluoropropylene

Abstract

X-ray diffraction has bcen used to m easure the separation of the molecular axes as a function of temperature, lamella thickness, and comonomer concentration in copolymers of tet ra fluoro ethylene and h exafluoropropylene. These data show that the increase of separation with increasing concentration of p erfluoromethyl groups is a consequence of inclusion of the gr\lups in the crystals a~d not an artifact associated with la mella thickness or crystal tranSitIOn temperature. ThIS conclUSIOn lS supported by the fact t hat diffuseness of some of the x-ray refl ections in the copolyme rs indicates the presence of molecular di sorder which might be ex pected from inclusion of the p erfluoromethyl groups in the crystals. The la mellas in the copolymers are thinner than those in the typical homopolymer a nd this aspect of the s tructure is the primary cause of the lower density in the copolym ers. Analysis of the availa ble data indicates t hat this is a lso t he situation in ?opolymers of ethyle ne a nd propylene. For both copolym er sen es, the de nSity of the lamellas IS apparently increased by thc inclusion of the m ethyl gro ups in t he cry stals as defects.

Theory of flow-induced fibril formation in polymer solutions

“…In all these instances, the expansion is essentially uniform in the sense that the increase in cross-sectional area is the same throughout the crystal, i.e., these systems behave as if they undergo equally distributed volume strain resulting from the presence of the end surfaces. It is also known that point defects such as -CF3 branches in poly(tetrafluoroethylene) have the general effect of expanding the entire lattice as has been shown by Bolz and Eby [17]. The latter point in particular shows that defects which are quite local in character can lead to relatively uniform expansion effects over considerable distances in systems consisting of chain molecules.…”

“…We begin th e ac tu al treatm ent of cu mulati ve stra in by defi ning E = excess free energy of c ilium or c ili ar y brid ge (e rgs/c iliu m) (17 ) a nd v = number of c ilia or ciliary bridges per unit area (number /c m 2 ) (18) so that VE is in the units of surface free energy. This defines leading to the general result that embodies the cumulative the constant C in eq (13) so we have strain approximation…”

Crystallization of random copolymers

“…A second copolymer with composition X2 (X2 > Xd will crystallize during similar quenching with an nverage temperature T'J. which is less than Tlo eqs (13) or (18) Now the temperature coefficient of AG* is negative (S decreases with decreasing T) and the effect of diffusional transport is to lower even farther the temperature (T2) required to obtain equal growth rates. Usually a WLF type of free energy is employed for AG*: (21) where Cl and C2 are positive constants and Til is the glass transition temperature of the polymer [8,9].…”

“…conditions (quenching of thin films of melt in ice water [13]) or at uniform cooling rates. During cooling most of the crystal growth will occur over a small temperature interval; say the average temperature in this interval is Tl for a copolymer of concentration XI.…”

X-ray investigations of the formation of core-fibrils crystallized out of the oriented melt

“…This can lead to relatively uniform expansion effects in the crystals [10,16]. In particular, an expansion effect can be expected arising from the ciliary bridges.…”