The mechanisms of the secondary crystallisation process in polymers: A narrative review

Secondary crystallisation in polymers relates to the formation of crystalline material not directly involved with primary crystal growth. Opposing viewpoints exist over whether this event occurs concurrently with primary crystallisation or subsequent to the impingement of spherulites. However, the development of additional crystallinity can alter the thermal and mechanical properties of polymers, highlighting the importance of this phenomenon. Historically, secondary crystallisation has been identified using a range of characterisation techniques, though there is considerable debate over the exact behaviour of the two main mechanisms involved: lamellar thickening and lamellar infill. The log(time) dependence of secondary crystallization and its timescale compared to the primary process are widely acknowledged in the literature, however many proposed mechanisms for lamellar thickening remain unsupported by experimental evidence, leading to a wealth of theoretical models. In contrast, while a more unified mechanism for lamellar infill has been established, multiple models have been employed to describe the spatial occurrence of this phenomenon within the spherulite structure. This review explores the mechanisms through which secondary crystallisation can occur and reveals the inconsistent nature of this phenomenon across different polymer systems.