Research on teacher professional learning communities (PLCs) has found that part of their benefit comes from increased collegiality. And yet, while the nature of such collegiality is theoretically multifaceted and includes a combination of instrumental, expressive and substantive relationships, this multidimensional nature of PLCs is almost never studied directly. In this paper, I fill that gap by examining the dynamics of relationship formation and evolution in a multiplex social network of 70 teachers during the first year of a small school reform. Using longitudinal social network models I analyse the within and between network dynamics of three types of discussion networks: course-related, personal, and, reform-related. Results show that the formation and evolution of ties within particular discussion networks were shaped by preference for similarity in demographic characteristics and attitudes. They also show that co-evolution between networks was shaped by reform-related discussions operating as a bridge connecting personal and course ties.