Free volumes and structural relaxations in diglycidyl ether of bisphenol-A based epoxy–polyether amine networks Pushkar N. Patil, a Sangram K. Rath, b Sandeep K. Sharma, a K. Sudarshan, a P. Maheshwari, a M. Patri, b S. Praveen, b P. Khandelwal b and Pradeep K. Pujari * a Two types of polyether diamines were used to prepare model rubbery and glassy epoxy–amine networks with diglycidyl ether of bisphenol-A; a,u-diamino terminated polyoxypropylene (POP) diamines and a,u- diamino terminated poly(oxypropylene)-block-poly(oxyethylene)-block-poly(oxypropylene)s (POP-POE- POP). The structural relaxations in the glassy and rubbery epoxy–amine networks at segmental (a relaxation) and local (b relaxation) levels were investigated by modulated differential scanning calorimetry (MDSC) and dynamic mechanical analysis (DMA). The characteristic length of glass transition of the networks x(T g ) was evaluated from MDSC using Donth's thermal fluctuation approach. While the POP diamine networks showed x(T g ) values of 2.0 and 2.07 nm, for POP diamine molecular weights of 230 and 400, respectively, the corresponding values for POP-POE-POP diamine networks were found to be 1.41 and 1.58 nm for POP-POE-POP diamine molecular weights of 600 and 900. This implied diminishing size of the cooperatively rearranging regions with decreasing crosslink density. DMA measurements were used to evaluate the crosslink density of the networks, characteristic features of the a and b transitions in terms of the width, intensity of transitions, and activation energy of the b relaxation. The studies revealed highly cooperative sub-T g b relaxations for the glassy networks and a truncated but pronounced b relaxation for the rubbery networks. Positron annihilation lifetime spectroscopy (PALS) was used to characterize the molecular topology of the networks in terms of the free volume nanohole sizes and their distribution. The difference of the average distance between crosslink points and the free volume nanohole size was seen to increase with the chain length of the diamines, indicating the fluctuational nature of the networks influenced by the sub T g relaxation. I Introduction Crosslinking of polymers, treated as a chemical modication, has greatly enhanced their practical applications. For example, crosslinked epoxy resins show excellent properties compared to linear polymers and hence nd diverse utilizations in composites, adhesives, coatings, etc. 1,2 Among them, epoxy– amine networks based on diglycidyl ether of bisphenol-A (DGEBA) and aliphatic/aromatic diamines have attracted the particular attention of researchers. 3–5 One of the outstanding features of epoxy–amine networks is the ease of tunability of the network architecture by varying the crosslink density and/or the exibility of chains between crosslinks. 6 Some of the common methods used to systematically vary the crosslink density of epoxy–amine networks are controlling the extent of cure, 7,8 altering the stoichiometric ratios of epoxy and amine, 9,10 using a mixture of monoamines and primary diamines, 11–15 and using aliphatic epoxy resins and diamines instead of conventional aromatic co-monomers 16 etc. Three dimensional epoxy–amine networks are characterized by a complex topology with regard to the spatial conguration of the molecular segments in the network. 17 Crosslinks are known to modify the relaxation behavior of the polymer networks through the introduction of network junctions, which in turn reduce the congurations available to the networks. 18–20 These network junctions, which are dependent on the chemical structure, affect the time and temperature dependence of macroscopic relaxation properties. 21,22 Theoretically, the effect of crosslinking on the glass transition temperature (T g ) is one of the fundamental indicators of the effect of crosslinking on the properties of the polymer. Experimentally, it has been shown that T g invariably increases with increase in crosslink density. 13,18,23,24 In addition to the T g , which depends markedly on the chain exibility and crosslink density, epoxy–amine networks are also characterized by the occurrence of strong sub- T g relaxations. 25–29 A number of studies have been performed to examine the effects of crosslinking on the local sub-T g a Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai – 400 085, India. E-mail: pujari@barc.gov.in; Fax: +91-22-25595151; Tel: +91-22-25595326 b Naval Materials Research Laboratory, Shil-Badlapur Road, Ambernath – 421 506, India Cite this: Soft Matter, 2013, 9, 3589 Received 2nd November 2012 Accepted 30th January 2013 DOI: 10.1039/c3sm27525f www.rsc.org/softmatter This journal is ª The Royal Society of Chemistry 2013 Soft Matter , 2013, 9, 3589–3599 | 3589 Soft Matter PAPER