Citation: John, J.; Klepac, D.; Kurek, M.; Šˇ cetar, M.; Gali´ c, K.; Vali´ c, S.; Thomas, S.; Pius, A. Phase Behavior of NR/PMMA Semi-IPNs and Development of Porous Structures. Polymers 2023, 15, 1353. https:// doi.org/10.3390/polym15061353 Academic Editor: Shaojian He Received: 6 February 2023 Revised: 3 March 2023 Accepted: 5 March 2023 Published: 8 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Phase Behavior of NR/PMMA Semi-IPNs and Development of Porous Structures Jacob John 1,†,‡ , Damir Klepac 2,3,† , Mia Kurek 4 , Mario Šˇ cetar 4 , Kata Gali´ c 4 , Sre´ cko Vali´ c 2,3,5, *, Sabu Thomas 6, * and Anitha Pius 1, * 1 Department of Chemistry, Gandhigram Rural Institute, Dindigul 624302, Tamil Nadu, India 2 Department of Medical Chemistry, Biochemistry and Clinical Chemistry, Faculty of Medicine, University of Rijeka, Bra´ ce Branchetta 20, HR-51000 Rijeka, Croatia 3 Centre for Micro- and Nanosciences and Technologies, University of Rijeka, Radmile Matejˇ ci´ c 2, HR-51000 Rijeka, Croatia 4 Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, HR-10000 Zagreb, Croatia 5 Rudjer Boškovi´ c Institute, Bijeniˇ cka 54, HR-10000 Zagreb, Croatia 6 School of Chemical Sciences, Mahatma Gandhi University, Kottayam 686560, Kerala, India * Correspondence: valic@irb.hr (S.V.); sabuthomas@mgu.ac.in (S.T.); dranithapius@gmail.com (A.P.); Tel.: +385-1-4571243(S.V.); +91-481-2730003 (S.T.); +91-451-2452371 (A.P.); Fax: +385-1-4680-084(S.V.); +91-451-2453071 (A.P.) † These authors contributed equally to the work. ‡ Present Address: Nebraska Center for Materials and Nanoscience (NCMN), University of Nebraska-Lincoln, 855 North 16th Street, Lincoln, NE 68588-0298, USA. Abstract: In this research, the porous polymer structures (IPN) were made from natural isoprene rubber (NR) and poly(methyl methacrylate) (PMMA). The effects of molecular weight and crosslink density of polyisoprene on the morphology and miscibility with PMMA were determined. Sequential semi-IPNs were prepared. Viscoelastic, thermal and mechanical properties of semi-IPN were studied. The results showed that the key factor influencing the miscibility in semi-IPN was the crosslinking density of the natural rubber. The degree of compatibility was increased by doubling the crosslinking level. The degree of miscibility at two different compositions was compared by simulations of the electron spin resonance spectra. Compatibility of semi-IPNs was found to be more efficient when the PMMA content was less than 40 wt.%. A nanometer-sized morphology was obtained for a NR/PMMA ratio of 50/50. Highly crosslinked elastic semi-IPN followed the storage modulus of PMMA after the glass transition as a result of certain degree of phase mixing and interlocked structure. It was shown that the morphology of the porous polymer network could be easily controlled by the proper choice of concentration and composition of crosslinking agent. A dual phase morphology resulted from the higher concentration and the lower crosslinking level. This was used for developing porous structures from the elastic semi-IPN. The mechanical performance was correlated with morphology, and the thermal stability was comparable with respect to pure NR. Investigated materials might be interesting for use as potential carriers of bioactive molecules aimed for innovative applications such as in food packaging. Keywords: interpenetrating networks (IPN); morphology; macroporous polymers; electron spin resonance/electron paramagnetic resonance ESR/EPR-spin probe; delivery of bioactive molecules; novel food packaging 1. Introduction Porous materials are the focus of much research due to a number of excellent properties and application areas [1,2]. Various applications need a unique set of pore characteristics of the biopolymer network since the different morphology of created pores contributes to numerous performances of created biopolymer networks [3]. Single network hydrogel Polymers 2023, 15, 1353. https://doi.org/10.3390/polym15061353 https://www.mdpi.com/journal/polymers