A comparative DSC, X-Ray and NMR study on the crystallinity of isomeric aliphatic polyamides

Abstract

In the polyamides 4.12, 10.6, 8.8 and 6.10 three fractions are present. The WAXD crystallinity is comparable to that of DSC and amounts to 40% in all samples. The rigid fraction obtained by solid state NMR is, however, close to 60%, pointing at 20% rigid amorphous material. A combination of the NMR, WAXD and SAXS results reveals that the latter fraction is situated inside the crystalline phase and not at the crystalline -'mobile amorphous' interface. Accordingly, one third of the crystalline layers with a thickness between 50 and 56 Å is not truly crystalline. From the width of the WAXD 100 reflections it can be deduced that crystalline grains exist with lateral dimensions of approximately 100 Å . Hence a picture emerges of alternating solid and liquid-like layers with the former consisting of crystalline grains separated by rigid amorphous matter.

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“…The distribution of phases α and γ analysed in this case is slightly different from that described in Ref. [18,19]. In the quoted publications, the contribution of phase γ was more visible, while the addition of clay had a significant influence on the initiation of creating a phase that would be less thermodynamically stable.…”

“…Phase α is formed by arrangement of chains in regular conformations (all trans), while phase γ is a quasi-hexagonal structure form, in which the chains are helically twisted due to an irregular arrangement of intermolecular hydrogen bonds between amide groups. The meta-stable phase β is a rare intermediate phase from phase α to γ, or vice versa [17][18][19]. In Fig.…”

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“…The peak positions of the fitting procedure and the spacings as calculated by Bragg's law are reported in Table 3. Within the experimental inaccuracy, the spacings do not show any significant differences according to the literature for PAs [18,19,24,40]. The amorphous regions are reflected on the smectic peak at 2h between 18°and 20°and the amorphous halo at ca.…”

“…For semicrystalline materials, a two-phase model of the structure was proposed with a crystalline and an amorphous part, but also a third phase can exist. This phase, the so-called rigid amorphous fraction (RAF), is located at the interfaces between the crystalline and amorphous regions, and is characterized by intermediate properties between two main phases and reduced mobility [18].…”

“…There are two types of hydrogen bonds that may occur in polyamidesthe amide-amide bond and the amide-ester hydrogen bond. Crystals of polyamides were found to be chain-folded and arranged in sheets held together by hydrogen bonds [18]. There are two possible hydrogen-bonded schemes for chain-folded sheets of PAs-the alternating and the progressive one, and they can stack with progressive shearing, yielding the so-called a-phase or with alternating shearing, which is known as the b-phase.…”